1. Field of the Invention
This invention relates to plug-in communication cards. More particularly, it relates to an interlock for a line-powered communication card which prevents the communication card from interfacing directly to the PSTN or other network when connected to a cellular device.
2. Background of Related Art
A modem is a device that generally transmits data between two processors. Modems are most frequently associated with a connection through a Plain Old Telephone Switch (POTS), otherwise known as the Public Switched Telephone Network (PSTN).
Communication card modems are available for many portable type devices such as laptop personal computers (PCs), and personal digital assistants (PDAs). One popular standard form-factor for a communication card modem is a Personal Computer Memory Card International Association (PCMCIA) Type I, II or III card, or PC Card, that plugs into virtually all portable computers and personal digital assistants (PDAs). Of course, multiple other standards exist, to which the present invention applies equally.
A PCMCIA card is a small, credit card size device which can generally contain anything from additional random access memory (RAM) to fax/modems to network interfaces. PCMCIA slots which accept a PCMCIA card are present on most new portable computers and many PDAs. Many different communication protocols may be present in PCMCIA communication card modems, either separately or in combination.
Initially, most PCMCIA or similar form communication card modems were designed to be connected only through a landline connection to the PSTN using an RJ-11 or RJ-45 telephone jack. U.S. Pat. No. 5,183,404 describes various configurations of what is commonly referred to as an `XJACK`. The term XJACK is believed to have been derived from the extendibility of the RJ-11 telephone jack generally only in the x-direction. One embodiment of that patent is shown herein in FIG. 5.
In FIG. 5, an RJ-11 telephone jack 86 or similar type connector is formed within an XJACK extension arm 72. A guide track 96 is formed within the communication card modem 70 to allow the XJACK extension arm 72 to be either in a retracted position enclosing the RJ-11 telephone jack 86 within the case of the communication card modem 70, or in an extended position as shown in FIG. 5 with the RJ-11 telephone jack 86 being accessible outside of the main perimeter 71 of the communication card modem 70. To interconnect a cable between the PSTN and the XJACK extension arm 72 of the communication card modem 70, the XJACK extension arm 72 must be in an extended position as shown in FIG. 5. A cable cannot be inserted into the RJ-11 telephone jack 86 when the XJACK extension arm is in the retracted position.
An alternative to the XJACK is a ZJACK as disclosed in U.S. Pat. No. 5,499,923. The term `ZJACK` refers to the rotatability of the RJ-11 connector in an extended position. FIG. 6 herein depicts one embodiment of the ZJACK as disclosed in that patent, wherein the RJ-11 or similar telephone jack 42, 46 is located at the end of a shank 22. The ZJACK is not only extendible in the x-direction, but is also rotatable out of the plane of the communication card 12 while in the extended position.
With the rise in popularity of both cellular telephones and portable computing devices such as PCs and PDAs came the need to allow interconnectivity between the communication card modem of a portable computing device and a cellular telephone for the purpose of transmitting data over the cellular network. Initially, cellular telephones were not capable of being plugged directly into the communication card modem of a PC or PDA designed for interconnection only to the PSTN. For instance, most cellular phones don't provide a dial tone required by many modems, or a ring voltage when the phone rings. Thus, cellular telephones did not include an RJ-11 or similar type telephone jack for direct connection to the PSTN.
To solve this need, a cellular data interface was developed to allow interconnection between a communication card modem of a PC or PDA, and a cellular telephone. To allow such interconnection, the cellular data interface generally includes circuitry for providing a dial tone and a ring voltage to the communication card modem. Because of this added circuitry, a separate connection in a communication card modem has been established for direct connection to a cellular telephone. This separate connection purposely is not an RJ-11 type jack, to prevent inadvertent connection of the PSTN to the cellular data interface. Instead, the connection of the cellular data interface in a communication card modem is generally a recessed socket on an edge of the communication card modem, in addition to the XJACK. In this way, the communication card modem has the capability to connect either directly to the PSTN or directly to a cellular telephone, depending upon which socket on the communication card modem is used.
Many portable computer devices such as PCs and PDAs utilize modems for communications purposes. However, battery size and weight is an important consideration in the design and convenience of such devices. A balance must be reached between the size and weight of the overall device, dictated largely by the choice of battery, and an acceptable length of operation between charges.
Unfortunately, although operating an acceptable length of time when running typical application programs, portable computer devices utilize a large amount of power when communicating over a telephone line via a modem. The battery source which powers both the portable computer device and its modem is typically sized for general computer applications, and runs out of power quickly when actively communicating over a telephone line through a modem. Portable computer devices such as PCs, PDAs, and hand held PCs (HPC) are designed to operate up to several hours on a single battery charge, but operate only fractions of an hour on a single battery charge when communicating via modem. Thus, although portable computer devices operate a sufficient length of time for quick data transfers over a modem powered completely by a battery, they require that external ac power be applied to allow for longer uses of the modem. It is therefore desirable for battery powered computer devices including a modem to draw power, in addition to the inherent battery, from a second power source.
The DC power inherent in a telephone line provides a convenient source of power, but there are often limitations and restrictions which limit the ability of a modem to derive power from the telephone line. For instance, present regulation in the United States is such that significant current may only be drawn from the telephone line when the telephone or modem is off hook or active. In order to hold the telephone line in an off hook condition, current in the approximate range of 20 mA to 150 mA must be drawn. The maximum amount of current available from the telephone line is also limited.
Communication card modems which are powered at least in part by power drawn from the telephone line have been developed to conserve the battery power of portable computing devices such as a PC or PDA. These line-powered communication card modems extended significantly the service time of the portable device before the battery of the PC or PDA required recharging. However, line power is available only when the communication card modem is connected to the PSTN. Cellular telephones do not always provide power to a telephone line for use by the communication card modem. Therefore, when connected to a cellular telephone, even the line-powered communication card modem must be powered externally.
There is a need for a reliable mechanism to allow a communication card modem to utilize power from the PSTN when connected to the PSTN, and to allow the communication card modem to be powered from an external source such as the PC or PDA as necessary, e.g., when connected to many types of cellular telephones. There is also a need to prevent the erroneous condition of having an interconnection from the communication card modem both to the PSTN and to a cellular telephone at the same time.